Present Doppler systems for measuring fluid motion use continuous wave techniques having separate transmit and receive elements or pulsed Doppler techniques with a single transducer time shared between transmit and receive elements. The continuous wave Doppler is sensitive along its entire sonic beam, while a pulsed wave Doppler is sensitive only along a sample volume whose length along the beam acts as a function of the receive gate width. Normally pulsed wave Dopplers use narrow gate widths, thereby sampling portions of a vessel in the case of blood flow measurement in a human subject, but the gate width can be increased to cover the entire vessel of interest.
Pulsed wave Doppler systems unfortunately experience the problem of "aliasing" when the Doppler shift exceeds one-half the pulse repetition frequency. Aliasing is a term from Sample Data Theory developed by Dr. Nyquist and Dr. Shannon and represents the signal confusion arising if the Doppler shift frequency exceeds one-half of the pulse repetition frequency. Some attempts have been made to increase the pulse frequency by sending two or three signals before previously transmitted and reflected (but not yet received) signals return. Systems of this type are commercially available and are referred to as "high pulse repetition frequency Dopplers."
Ultrasonic Doppler flow meters for the measurement of blood flow in a patient's circulatory system are well-known and illustrated in numerous prior art patents such as U.S. Pat. No. 3,430,625 (McCloud); U.S. Pat. No. 3,888,238 (Mindel et al); U.S. Pat. No. 3,554,030 (Peronneau); U.S. Pat. No. 3,766,517 (Fahrbach); U.S. Pat. No. 4,067,236 (Hottinger); U.S. Pat. No. 3,977,247 (Hassler); U.S. Pat. No. 3,498,290 (Shaw); and U.S. Pat. No. 3,987,673 (Hansen). All of these references are incorporated herein by reference to show the conventional methods and circuitry to accomplish those methods.